ci: pin the bare-metal nightly lane past a clippy ICE

[JustinKovacich]

Bare-metal runtime (nightly) has been red on main since 2026-08-22 with no code change behind it.

What breaks

Every nightly from nightly-2026-08-22 onward ICEs the job:

error: internal compiler error: unexpected rigid alias in layout_of after normalization:
  Alias(Yes, Alias { kind: Opaque { def_id: DefId(0:2189 ~ simple_someip[64f0]::bare_metal_tasks::run_someip::{opaque#0}) }, ... })

It is the toolchain, not us

• Plain cargo +nightly check -p simple-someip --no-default-features --features bare-metal-runtime,client is clean on the same nightly — only cargo clippy fails.
• Reproduces on unmodified main; re-running the job on main's last green commit (6369f1c, green 08-21) fails today.
• Bisected locally: nightly-2026-08-21 (rustc 8925ea358) is good, nightly-2026-08-22 (rustc c656540d6) is the first bad one, and -23 / -24 stay bad.

The trigger is clippy::pedantic enabling large_futures, which switches to Codegen mode to read a coroutine layout while the opaque aliases are still marked rigid from type checking. Reported upstream several times over (rust-lang/rust#161542, rust-lang/rust-clippy#17622, #17617) and fixed by rust-lang/rust-clippy#17601, merged 2026-08-23 — but that fix has to ride a clippy subtree sync into rustc before it reaches a nightly, so the lane stays red until then.

Why pin rather than -A clippy::large_futures

Pinning keeps the lint running. It is worth more on this lane than anywhere else in the tree: a fat future on the bare-metal runtime is a static pool sized to hold it. Allowing the lint would silence it everywhere the runtime is linted, for however long the sync takes.

Verification

All four steps of the job pass locally on nightly-2026-08-21:

Step	Result
clippy … bare-metal-runtime,client	pass
clippy … bare-metal-runtime,server	pass
build -Z build-std=core --target thumbv7em-none-eabihf (client, server)	pass
cargo doc under RUSTDOCFLAGS=-D warnings	pass

The other nightly job (build-std core gate) runs no clippy, is unaffected, and is left floating deliberately.

Unpinning

The comment on the step says to restore @nightly and delete it once a nightly carries the clippy fix. Worth checking back in a week or so.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

@@           Coverage Diff           @@
##             main     #152   +/-   ##
=======================================
  Coverage   81.30%   81.30%           
=======================================
  Files          47       47           
  Lines       15514    15514           
=======================================
  Hits        12613    12613           
  Misses       2901     2901